Does anyone know whether having well formed, valid markup, makes a difference to search engine rankings? I only ask because I've been working on a site recently that looks great but I've not declared a DOCTYPE and some tag attributes are not supported by by W3C standards - such as 'LEFTMARGIN=0' which is always useful. I was just wondering if anyone had noticed a difference with fully validated markup against, well, not so fully validated markup?
It won't make much (if any) difference unless your markup is bad to the point that the SEs can't figure out what is text and what is code in your HTML. But there are some other good reasons for valid code.
Valid markup won't help at all. The only important thing for seo is properly nested tags, so that the search engine can parse the file properly.
Agreed with the statements above -- especially about making your code as easy to read as possible for the spiders. Try to get your lines of content as high up in the code as possible... don't have all your javascript and css in the first 100 lines of code - make those external sheets, etc. etc.
and you can do a left margin in your CSS anyway and its quite simple to put a doctype in, so theres no reason why you cant be valid.
Okay, I'm ready for the flak, bad reps and critics. I say the answer is YES ... okay, and no ... Building on tflight's comments: Not in the sense that it improves SERP or it's necessary to getting indexed, ranked, etc. but that in the sense it ensures you don't have any "show stoppers" that could be damaging to all of these issues. If you have an unrealized bad code error that prevents a bot from crawling your site and following on to other pages - YES, it can make a difference; and, I see this all the time, people complaining that "x" engine won't index/cache their site only to find out they've some critical, but easy to fix error that is preventing the engine from getting pass line 10 of their code! To use Google as an example is like asking your parents why they can do something that you're not allowed to do ... it's good to be the king/boss! Again, it generally won't make a squat difference if your code is W3C valid as far as getting indexed. BUT, if you code is invalid with a critical error how well do you expect to be listed in SERP? Also, I would argue that it is more important to SEO than others have stated. It's vital not just for properly nested tags, but as a "second set of eyes" to ensure that you haven't improperly or mistakenly duplicated an id, forgotten to declare an alt-text for an image (even if declared <empty>) and other issues. Additionally, at W3C you can run "link checker" to ensure you haven't duplicated an anchor text - a big no-no for Google and others. So anyway, no it's not necessary for SERP, but it could be if your code isn't simply valid, but has critical errors. On a side note, I see no reason not to have valid code. It helps ensure accessibility and ease of bot crawling, as well as acting as a good second opinion / set of eyes to make sure you haven't overlooked something. Further, it helps ensure web accessiblity for users. <noscript> and <frameset> coding can make a big difference in how potential users see - or don't see - your site. Another benefit of W3C validation, is the use of the "detailed" tool, specifically the "show outline" option that allows you to view your pages in the emphasis that many bots will see them. A good outline and headings use can make a huge and postive difference, while poor or excessive usage can be a proverbial shot in the foot. Okay, I'm ready for the flak ...
i think i would just about agree, its nice to have valid code the main one being (IMO) is that it is more accesiable to more viewers. plus if you ever went for a job in web design or whatever your code is like your CV/Resume if it has mistakes and errors who would hire you?
I don't think that search engines apreciate it, but even if it they do, seems to make no big difference, so I just don't try to validate my sites. That's true. Now that I think about it, having a look at my sites, I think I wouldn't hire myself.